U.S. Pat. No. 5,719,311, the content thereof is incorporated herein by reference, discloses a fixed bed catalytic process for the esterification of carboxylic acids and alcohols into carboxylic esters. In the process disclosed in the '311 patent, the fixed bed reactor contains acidic catalysts that are present in a solid phase, and the reaction condition is controlled such that (i) the reactants and the products co-exist in a gas-liquid two-phase equilibrium in the fixed bed reactor and that (ii) at least one component of the reactants is present in one phase and at least one component of the products is present in another phase. Very high reaction yield and selectivity, typically better than 90%, sometimes exceeding 99%, were observed with the process disclosed in the '311 for preparing the esters of Methanol/propionic acid, Methanol/methacrylic acid, isobutanol/hexahydrophthalic anhydride, and isooctyl alcohol/phthalic anhydride. However, when the carboxylic acid is acetic acid, which has high miscibility with many alcohols, the reaction yield from the process of the '311 will not be in the same high level as with other carboxylic acids, and conventional processes need to be used to produce esters of acetic acid in an economic manner. Lowered reaction yield causes unsatisfactory amounts of alcohol to remain in the production, thus, further adversely affecting the economic potential of the process, especially with regard to post-esterification purification cost.
Esters of acetic acids, or acetate, and derivatives thereof have been used in a wide variety of industrial applications, such as for use in making coatings, adhesives, perfumes, plasticizers, etc. Unsaturated carboxylic esters can also be used as monomers or intermediate raw materials in preparing resins covering a wide range of applications.
Conventionally, the processes of making acetic esters from acetic acids and alcohols can be classified into the following three main categories:
(a) Liquid-phase esterification reaction utilizing a liquid catalyst: This type of processes utilize liquid phase acid, such as sulfuric acid, phosphoric acid, sulfonic acid, or p-toluenesulfonic acid, as catalysts. PA1 (b) Liquid phase esterification reaction utilizing a solid catalyst: This type of processes typically utilize a cationic ionic exchange resin as catalyst. Examples of this type of processes include those disclosed in Japan Laid-Open Patent Application 2-279654 and European Patent EPO-10,953. PA1 (c) Gas phase esterification reaction: This type of processes utilize a variety of catalysts such as heteropolyacids (Japan Laid-Open Patent Application 57-99556), oxides (Japan Laid-Open Patent Application 51-76019), liquid phase acids carried by a solid carrier (UK Pat. No. 1,017,806; U.S. Pat. No. 5,151,547; Japan Laid-Open Patent Application 43-20286), and zeolite (SU 1719393) in a gas phase reaction.
One of the problems associated with the liquid-catalyst liquid-phase esterification reaction is that the acidic liquid catalysts of sulfuric acid or p-toluenesulfonic acid can cause corrosion problems to the reactor. These liquid acid catalysts are also discharged along with the reaction products, thus causing severe waste disposal and pollution problems. Furthermore, because the esterification of acetic acids involves a reversible reaction, in order to increase the conversion rate of acetic acids, either excessive amounts of alcohols must be used, or the product from the esterification reaction must be constantly removed from the reaction system. In either case, the production cost of carboxylic esters is substantially increased.
The solid-catalyst, liquid-phase esterification reaction, which typically utilizes a cationic ionic exchange resin as catalyst, ameliorates the corrosion and waste disposal problems experienced with the liquid-catalyst liquid-phase processes, and results in simplified separation procedure required between the reaction product and catalysts. However, cationic ion-exchange resins typically exhibit relatively poor heat-resistance, and they often lose substantial activity after being subject to heat. Once the catalytic activity of the cationic ion-exchange resins is reduced, it is difficult to be regenerated. Furthermore, during the solid-catalyst, liquid-phase esterification process, reaction products cannot be removed from the reaction stream so as to favorably change the reaction equilibrium, and the reaction yield can only be improved by separating unreacted reactants from the product stream and recycling the unreacted reactants. This causes the production cost to be maintained at a relatively high level.
In the gas phase esterification reaction, the reaction conditions are maintained so that all the reactants and products are in the gas phase. Typically, inorganic materials are utilized as catalysts which typically exhibit excellent heat resistance and can be easily separated from the reaction products. However, the gas phase reaction necessitates a relatively large reaction vessel, resulting in large capital investment cost. Furthermore, if the gas phase esterification reaction is utilized to produce unsaturated carboxylic esters, the high reaction temperature often causes undesired by-products of polymers or oligmers to be produced. In certain instances, the high reaction temperature has caused the alcohol molecules to be dehydrated to become ethers. These side-reactions will tend to cause the reaction catalysts to lose their activity and result in operational difficulties.
The process disclosed in the '311 patent solved many of the problems described above. However, as it is currently structured, the process disclosed in the '311 patent is not very economically attractive, and hence is not commercially applicable for the esterification of acetic acid.
U.S. Pat. No. 4,939,294 discloses a catalytic distillation process for the preparation of methyl acetate which utilizes sulfuric acid a catalyst and which requires a 72-tray catalytic distillation tower. The catalytic distillation process offers a feasible alternative to the conventional process for making methyl acetate, because the latter typically requires eight distillation towers for post-esterification separation. For ethyl acetate, which only requires three distillation towers for post-esterification purification in the conventional processes, the catalytic distillation process disclosed in the '294 using sulfuric acid as catalyst, is no longer attractive.